We propose to examine the mechanism of action of eicosapentaenoic acid (EPA; 20:5 (n-3)) in platelets of normal men and in a select group of hypercholesterolemic patients. We will test the hypothesis that EPA effects are to a major degree mediated by biophysical changes in the platelet membrane. We postulate that the lipid matrix of the platelet membrane contains areas of inhomogeneity in which unsaturated fatty acyl residues of 2 and 3 double bonds are more prevalent. These areas may be sites at which pseudopodia may be extruded. EPA enrichment of platelets will replace some of these unsaturated fatty acids and thus reduce the number of pseudopodia formed. Our preliminary experiments support this hypothesis in showing that pseudopodia have a completely different phospholipid distribution compared to platelet bodies and that platelets obtained from subjects on a dietary fish oil supplement for 6-8 weeks show markedly reduced numbers of pseudopodia upon agonist stimulation. The effects of EPA on platelets will be assessed by: 1) monitoring the number of pseudopodia extruded upon stimulation 2) analyzing lipid composition in isolated pseudopodia and platelet bodies, 3) evaluating membrane fluidity using fluorescent polarization and electron spin resonance probes, 4) evaluating cyclic nucleotide and prostanoid metabolism in these platelets components, 5) determining the presence of glycosaminoglycans on pseudopodia and platelet bodies and 6) correlating these parameters to platelet function as manifested by aggregation and adhesion (measured in a flow chamber). The role of membrane cholesterol and its relation to the effect of EPA will be examined in a group of hyper-cholesterolemic patients with a variety of dyslipoproteinemias. EPA will be administered in doses of 3, 6, and 9 g/day, generally for 3 weeks each, either as a purified preparation of the fatty acid or as a fish oil concentrate. We propose 2 controls for this study, one pre-and post EPA administration, the other a group of normal subjects on corn oil supplements. These studies provide information on the role which the bio-physical structure of EPA plays in mediating platelet function and enhance our understanding of the platelet response to agonists.